It is very common for rechargeable battery assemblies (battery packs) which are used in portable communication devices, such as two-way radios, to include a reverse current protection diode. This reverse current protection diode is used to prevent current from flowing out of the B+ terminal if the battery is put into a charger and the charger is turned off. Battery protection diodes are particularly important in flammable environments since they prevent sparking in case of an inadvertent short on the battery's charger side B+ and B- contacts.
Referring to FIG. 1, there is shown a prior art battery charging scheme consisting of a charger 102, radio battery 106 and radio 104. Radio 104 contains positive (B+) and negative (B-) battery terminals which are coupled to radio battery 106 via battery contacts 116 and 114, respectively. Battery 106 contains one or more battery cells 108, which determine the voltage and current capacity of battery 106. Also included as part of the battery 106, is a reverse current protection diode (D1) 118, a battery temperature indicator such as thermistor (Rt) 112 and a battery capacity indicator, such as resistor (Rc) 110.
Charger 102 consists of a charger monitor circuit 128, which can consist of a well known microprocessor or microcontroller as known in the art and appropriate control software. Charger monitor circuit 128 controls charger control circuit 130 which provides current to battery 106 in order to charge the battery. Charger control circuit 130 is implemented using a well known programmable current source circuit. A control signal is transmitted by charger monitor circuit 128 to charger control circuit 130 via bus 140, the control signal informs charger control circuit 130 on how much current to source via line 129 to battery 106.
Charger monitor circuit 128 contains three analog to voltage on the B+ line. A/D port 122 senses the resistance of capacity resistor Rc 110 and A/D port 124 in turn senses the resistance of thermistor Rt 112, as its resistance changes once the battery begins charging. A/D ports 122 and 124 include external pull-up resistors which are used to determine the resistance of Rc 110 and Rt 112, by determining the voltage level at A/D ports 122 and 124, respectively.
A major problem with the battery and battery charging scheme shown in FIG. I is that protection diode 118 experiences a 0.71 volt drop if it is a silicon diode and a 0.52 volt drop if it is a Schottky diode when for example, two amperes of current are flowing from battery charger 102 to radio 104 or battery 106. The power dissipation for the silicon diode would be approximately 1.42 watts and for the Schottky diode would be 1.04 watts. These high power dissipation levels causes battery 106 to heat up excessively. This causes thermistor 112 not to measure the true temperature rise of battery cells 108. Which in turn causes charger 128 not to provide proper charging currents to be applied to battery 106. Since the heat rise in battery 106 is not only caused by battery cells 108, but by protection diode 118, a proper determination of when battery cells 108 become exothermic in order to shut off rapid charge currents to battery 106 can also not be accurately determined.
Another problem caused by protection diode 118 is that ultra fast charging levels (e.g., charging at a charge current equal to five times the capacity of battery 108, or other high charge currents) can not be used to charge battery cells 108 quickly given the enormous amount of power dissipation that would occur by the voltage drop of diode 118.
A final problem with the prior art battery charging and sensing scheme shown in FIG. I is that because of the reverse protection diode, the battery is not allowed to be discharged from the battery's charger side B+ contact for battery conditioning purposes. Thus, a need exists for a battery and method for charging a battery which can solve the above mentioned problems.